Conventional magnetic resonance imaging (MRI) methods rely on a linear relationship between magnetic fields and spatial positions of the nuclear spins within the object being imaged. This relationship is realized by using a combination of homogeneous magnets and linear gradient coils. Deviations from this linear relationship can cause undesired image distortions and sometimes artifacts.
Field anomalies are deviations of magnetic fields from prescribed values. Anomalies can be static as well as sequence-induced. Static anomalies may occur from, for example, field inhomogeneities produced by the magnet used to generate the main static magnetic field. Sequence-induced anomalies may occur from, for example, eddy-currents or the Maxwell fields.
Two remedies often used for correcting anomalies in magnetic field are: (i) minimization of inhomogeneities by shimming the magnetic field using special shimming hardware, and (ii) post data acquisition image correction to compensate for the effects of the field anomalies. These remedies require accurate information about the spatial distribution of the magnetic field at the time when the image data is acquired.
Conventionally, maps of the magnetic field are generated using dedicated field-mapping sequences. Since images are acquired using a different sequence, such field maps contain no information about sequence-induced field anomalies relevant to the imaging sequence. In cases when the sequence-induced changes are significant, such as during balanced steady-state free precession (SSFP) sequences, it is preferable that the field maps be generated using the exact same sequence that is used for imaging.
In addition, field maps are often generated from the phases of the acquired images. As anyone skilled in the art understands, phases of acquired magnetic resonance (MR) signals can be affected by factors other than the static magnetic field. Such factors include, but not limited to, chemical shifts and radio-frequency (RF) field orientation. For example, in the single quadrature Dixon (SVD) method recently introduced for water-fat separated SSFP, maps of magnetic field are obtained from the phases of the isolated echo images. Since the phases of the echo signals are influenced by field inhomogeneities as well as chemical shifts, the magnetic field maps so generated are susceptible to interference from chemical shifts.
There is therefore a need for improved methods to generate magnetic field maps that are relevant to imaging sequences, wherein the maps are not degraded by interferences such as that from chemical shifts.